The gas injection molding process created a great deal of interest when it was first introduced, especially on the part of the automotive plastics industry. The process allows injection molders to make larger parts with increased rigidity at lower clamping pressures. This, in turn, allows parts to be molded that have not previously been able to be created.
However, the process has been hampered by problems. First and foremost have been the numerous patent infringement suits and licensing difficulties that have retarded the spread of the technology in the United States. Second, technological problems - such as controlling the seemingly erratic nature of the gas - have also been an issue. As with any new molding technology, the plastics industry is still attempting to establish logical techniques to set up and rationalize processing conditions for the method. Although the gas equipment allows a great deal of adjustments to be made, there is an optimum processing window -- providing control and repeatability of the process -- that needs to be established in order to make consistent, acceptable parts.
The purpose of this paper is to describe one set of rationalization techniques for the emerging gas injection technology that is applicable regardless of the type of machine or processing license a molder is using. This technique makes use of an in-line pressure transducer and one or more in-mold pressure transducers to determine the actual gas injection pressure and, therefore, flow rates to optimize processing. The technique allows the comparison of input process conditions versus actual changes that affect the resultant part. The discussions will not be specific to any particular machine or material.